Sulphur may be used as an alternative for Portland cement as binder material in construction materials. Sulphur based construction materials, such as sulphur cement and sulphur concrete have distinctive advantages over their Portland cement based counterparts. Sulphur based construction materials are strong, acid and salt resistant and thus applicable in many fields of industry including those where contact with aggressive environments is anticipated. Typical fields of application include building blocks, bricks, tiles, floors, coatings, foundations, acid reservoirs, etc.
Sulphur cement is a thermoplastic material that typically melts at temperatures in the range of from 115 to 125° C., depending on its exact composition. Upon cooling, the liquid sulphur cement re-solidifies at temperatures below the melting temperature.
The ability to melt and subsequently re-solidify allows for the convenient shaping of sulphur cement. A typical process for shaping sulphur cement is moulding.
In CA2267860 is disclosed a process for preparing sulphur comprising concrete blocks, wherein a mixture of powdered sulphur and sand having temperature of 110° C. (230° F.) are moulded and compressed in a urethane mould.
FR2773340 discloses a process for preparing concrete elements using a mould. The mould is formed out of a deformable, flexible plastic material such as polyurethane, silicone or others. FR2773340 does not disclose the dimensions of the mould or its use at elevated temperatures. According to FR2773340, the obtained concrete elements show surface defects and irregularities similar to traditionally produced elements.
Moulded sulphur cement or concrete based structures can suffer a deterioration of compression and flexural strength due to the intrusion of water, as for instance shown in U.S. Pat. No. 4,256,499. Water intrusion occurs due a bad surface finish, which results in the presence of micro cracks and porosity on the outer surface of the structure. As a consequence, the compression strength of the sulphur cement or concrete structures decreases.
In U.S. Pat. No. 4,256,499 is disclosed a method of producing a shaped sulphur concrete article with a good surface finish. In the method of U.S. Pat. No. 4,256,499, adhesion to the mould is prevented by removing the mould prior to melting the sulphur. In the method of U.S. Pat. No. 4,256,499, a mouldable mixture comprising a mineral binding agent and a sulphur component is compressed in a steel mould under high pressure. The thus formed compressed shaped body of the mixture is removed from the mould and subsequently heated to a temperature sufficient to melt the sulphur and cooled to obtain a shaped article. The obtained articles showed post-production water intrusion, exhibiting a reduction in compression strength up to 20% after being soaked in water of 25° C. for 1 hour. The method of U.S. Pat. No. 4,256,499 requires the formation of a self-supporting compressed shaped body prior to melting the sulphur. Furthermore the method of U.S. Pat. No. 4,256,499 requires that the mould be removed prior to melting the sulphur.
In U.S. Pat. No. 4,981,740 is disclosed a method for producing sulphur concrete pipes using casting moulds. To prevent sticking of the sulphur concrete to the mould, U.S. Pat. No. 4,981,740 suggests to preheat the mould and to apply a release agent, i.e. form oil or a polytetrafluoroethylene (PTFE or Teflon™) coating on the surface of casting mould. Applicants have found that sulphur based objects moulded by the process of U.S. Pat. No. 4,981,740 have a rough surface.
There is a need in the art for a mould and an improved process for producing shaped objects of sulphur cement and/or sulphur concrete with a good surface finish, i.e. a smooth surface.